1. Field of the Invention
The present invention relates to a mechanism, hereinafter referred to as a W-mechanism, which includes a main shaft, a first member relatively rotatably mounted, but held against rotation, on the main shaft, an auxiliary shaft revolvable around the main shaft, and a second member rotatably mounted on the auxiliary shaft and revolvable around the first member, but held against rotation about its own axis, or which includes a main shaft, an auxiliary shaft fixed to the main shaft, a first member rotatably mounted on the main shaft, and a second member rotatably mounted on the auxiliary shaft, wherein the second member is rotatable in unison with the first member.
2. Description of the Prior Art
Known W-mechanisms include Oldham couplings and universal joints, for example. One typical apparatus which requires such a W-mechanism to be incorporated therein is a radial-plunger-type pump or motor. A hydraulically operated mechanical unit for use as such a radial-plunger-type pump or motor is disclosed in Japanese Patent Application No. 1-171637, for example, filed by the assignee of the present application. The disclosed unit will be described below with reference to FIG. 12 of the accompanying drawings.
As shown in FIG. 12, the unit comprises a main shaft (not shown) having an axis O1 and an auxiliary shaft 501 that is eccentric with respect to the main shaft and has an axis O2. The auxiliary shaft 501 is integrally coupled to the main shaft. When the main shaft rotates about its own axis O1, the auxiliary shaft 501 revolves around the axis O1. A coupling ring 503 is relatively rotatably mounted on the auxiliary shaft 501 through a bearing 502. To the coupling ring 503, there are connected a plurality of radial rockable plungers 505 through respective coupling pins 504 at circumferentially equal intervals around the axis O2 and a single radial fixed plunger 506 integral with the coupling ring 503. The unit also includes a cylinder casing 510 having an axis aligned with the axis O1 and rotatable about its own axis with respect to the main shaft. A plurality of cylinders 508 are rockably mounted on the cylinder casing 510 through respective supporting pins 511 at circumferentially equal intervals around the axis O1. The plungers 505, 506 are slidably fitted in the respective cylinders 508.
When the unit is to be used as a hydraulic pump, the cylinder casing 510, for example, is held against rotation, and the main shaft is rotated to cause the auxiliary shaft 501 to revolve around the axis O1. The coupling ring 503 also revolves with the auxiliary shaft 501 around the axis 01, causing the plungers 505, 506 to reciprocally move in the respective cylinders 508 for thereby drawing oil into and discharging oil out of the cylinders 508. Since the cylinder casing 510 is held nonrotatable, the coupling ring 503 only revolves around the axis O1, but does not rotate about the axis O2, in order to allow the plunger 505 to reciprocally move in the corresponding cylinder 508. More specifically, the coupling ring 503 and the auxiliary shaft 501 are relatively rotatable through the bearing 502, and the plunger 506 integral with the coupling ring 503 prevents the coupling ring 503 from rotating with respect to the cylinder casing 510.
With the above arrangement, when the main shaft rotates about its axis O1, the coupling ring 503 on the auxiliary shaft 501 revolves around the axis O1 while being prevented from rotating about the axis O2, thereby causing the plungers 505, 506 to reciprocally move in the respective cylinders 508.
Conversely, when oil under pressure is supplied into and discharged out of the cylinders 508, the main shaft is rotated about the axis O1. Therefore, the unit operates as a hydraulic motor.
The unit can also operate as a hydraulic pump or motor with the main shaft held against rotation. In this mode of operation, the cylinder casing 510 is rotated about the axis O1. Inasmuch as the plunger 506 is integral with the coupling ring 503, the rotation of the cylinder casing 510 causes the coupling ring 503 to rotate on the auxiliary shaft 501, which is held against rotation, at the same speed as the cylinder casing 510. The plungers 505, 506 are now caused to reciprocally move in the respective cylinders 508, thereby drawing oil into and discharging oil out of the cylinders 508. Consequently, the cylinder casing 510 and the coupling ring 503 rotate about the respective axes O1, O2 at the same speed, and the plungers 505, 506 reciprocally move in the respective cylinders 508 to draw oil into and discharge oil out of the cylinders 508.
In the radial-plunger-type pump of the above structure, when the coupling ring 503 rotates on the auxiliary shaft 501 with rotation of the cylinder casing 510 to cause the plungers 505, 506 to reciprocally move in the respective cylinders 508, the plunger 506 reciprocally moves with its axis always aligned with a straight line interconnecting the axis O2 and the center of the support pin 511 about which the cylinder 508 is angularly movable. During rotation of the coupling ring 503, the other plungers 505 reciprocally move in the corresponding cylinders 508 while rocking about the respective coupling pins 504, in order to permit the reciprocating movement of the plunger 506 integral with the coupling ring 503. Therefore, the plungers 505, 506 have slightly different strokes of reciprocating movement, and discharge oil at different instantaneous rates. More specifically, while the cylinder casing 510 is making one revolution, the plungers 505, 506 reciprocally move successively in respective strokes thereby to discharge oil under pressure from the respective cylinders 508. Because the plungers 505, 506 discharge oil at different instantaneous rates, the total rate at which the oil is discharged under pressure from the hydraulic pump fluctuates during one revolution of the cylinder casing 510.
FIG. 13 of the accompanying drawings shows one conventional unit, for use as a pump or a motor, which includes a slide ring 520 mounted on an auxiliary shaft 501, and plungers 525 slidably fitted in respective cylinders 508 and having ends 526 slidably held against an outer circumferential surface 521 of the slide ring 520. In this unit, since the plungers 525 reciprocally move in identical strokes upon rotation of the slide ring 520, the rate at which the oil is discharged under pressure from the unit, operating as a pump, is prevented from fluctuating during one revolution of the casing.
However, the conventional unit shown in FIG. 13 has a problem in that though the plungers 525 can be pushed radially outwardly, i.e., into the respective cylinders 508, by the slide ring 520, but cannot be pulled radially inwardly, i.e., in a direction out of the cylinders 508. Therefore, the unit shown in FIG. 13 is not suitable for use as a hydraulic pump unless the suction side of the pump is pressurized.
One solution is to use a mechanism, referred to as a W-mechanism, which includes a main shaft, a first member relatively rotatably mounted, but held against rotation, on the main shaft, an auxiliary shaft revolvable around the main shaft, and a second member rotatably mounted on the auxiliary shaft and revolvable around the first member, but held against rotation about its own axis, or which includes a main shaft, an auxiliary shaft fixed to the main shaft, a first member rotatably mounted on the main shaft, and a second member rotatably mounted on the auxiliary shaft, wherein the second member is rotatable in unison with the first member, the mechanism being interposed between the main shaft and the auxiliary shaft.
Known W-mechanisms include Oldham couplings and universal joints, for example, as described above. The Oldham couplings have an intermediate member reciprocally movable, causing a slip against the members coupled to the main and auxiliary shafts that are displaced out of coaxial alignment with each other. The greater the distance of eccentricity by which the main and auxiliary shafts are displaced out of coaxial alignment, the greater the slip. Therefore, if the distance of eccentricity is relatively large or the main and auxiliary shafts are required to transmit a high torque or rotate at a high speed, then the various sliding surfaces tend to suffer wear, seizure, and vibration. The universal joints are free from these drawbacks, but require a relatively large axial dimension. As the distance of eccentricity between the main and auxiliary shafts is larger, the axial dimension is also larger, making it difficult to combine the universal joints with radial-plunger-type pumps or motors.